advaita, vishishta-dvaita and dvaita ....
All these three gurus have interpreted the Upanishads in their own ways. Anciet Sages (Rishis) like Bhrigu, Vashishta and Yagnyavalkya contributed to the emergence of Upanishads. Their wisdom was collected in the Sanskrit scriptures called Upanishads. The main Upanishads are found at the end of every Vedas. That is why they are called the Vedantas. Plus Brahmasutras also contributed to these Upanishads too.
Shankara
Apart from these three, there are many interpretations like
Bhedabheda of Yadava Prakasha (10th century)
Dvaitadvaita of Nimbarka (13th Century)
Shuddhadvaita of Vallabhacharya (16th Century)
Achintya Behdabheda of Chaitanya Mahaprabhu (16th Century)